1. Field of the Invention
The present invention relates to an image sensor and more particularly to an image sensor which includes one or a plurality of first light emitting elements for irradiating an original with light, one or a plurality of photoconductive elements for sensing light via the original, and one or a plurality of second light emitting elements for irradiating the photoconductive elements with light.
2. Related Background Art
Recently, with the advance of electronic technology, the use of various image transmission and/or processing apparatus is becoming widespread. A device which converts an image in an original into an electric signal, namely, an image sensor, is indispensable for apparatus of this type. Usually, an image sensor is composed of photoelectric conversion elements which each convert an optical signal to an electric signal, a source of light for illuminating the original and an optical system, inclusive of non-imaging elements, for guiding the reflected light from the original to photoelectric conversion elements. As a photoelectric conversion elements, a charge coupled device (CCD) is often used. Although the CCD as such is of the miniature type, the optical system is a reducing optical system which is composed of large parts such as a mirrow and lenses. Thus these parts are an obstacle to miniaturization of the entire device. Thus, so-called contact type image sensors are being developed and are becoming widespread which include a photoelectric conversion element made of amorphous semiconductor such as amorphous silicon (a-Si), CdSe-CdSe, an array of LEDs and an imaging light converging element (trade name: SELFOC manufactured by Nihon Sheet Glass). Photoelectric conversion elements using amorphous semiconductors are classified into the photoconductive type, which permits injection of charge carriers out of the electrodes, and the photodiode type, which does not permit carrier injection. Generally, the photoconductive type provides high conversion efficiency and hence a large output signal, thereby resulting in a high-quality image signal. However, it shows slow response to changes in the optical input and has been considered to be unsuitable for high-speed operation. In order to compensate for the above drawback of the photoconductive type photoelectric conversion elements, a method called bias light irradiation is known. FIG. 1 shows one example of an image sensor using bias light irradiation. Herein, reference numeral 1 denotes a transparent insulative substrate; reference numeral 2, an amorphous semiconductor layer deposited on the substrate; reference numeral 3, opposing electrodes. A highly doped layer for carrier injection may be provided, as needed, between the opposing electrodes 3 and semiconductor layer 2. Semiconductor layer 2 and electrodes 3 constitute a photoconductive element. Reference numeral 4 denotes an IC for processing electric signals. A document 5 is irradiated with light by an array of LEDs 6. The reflected light from the original is caused to enter semiconductor 2 between opposing electrodes 3 by an imaging light converging element 7. It is arranged that the light from another array of LEDs 8 directly enters semiconductor layer 2. That is, the reflected light from the original and the light from the array of LEDs 8 enter semiconductor 2, which is referred to as bias light irradiation.
The effect of the conventional image sensor having the above structure and using bias light irradiation will now be described with respect to the waveforms of FIG. 2. Assume that original 5 is covered with alternate white and black stripes and that the document is fed in a given direction. FIG. 2(a) shows the intensity of the reflected light from the document in which reference numeral 201 denotes the light intensity obtained when the white stripe is being read and reference numeral 202 the light intensity obtained when the black stripe is being read. FIG. 2(b) shows a change in the photocurrent flowing out of the photoconductive elements when only the array of LEDs 6 is emitting light. FIG. 2(c) shows a change in the photocurrent flowing out of one of the photoconductive element when both arrays 6 and 8 are emitting light at the same time. It can be seen from FIGS. 2(b) and (c) that lighting of array 8 will expedite the response of a change in the photocurrent. Thus the original can be moved and read at higher speeds than otherwise.
It should be noted that in bias light irradiation, a photocurrent component 204 is contained in photocurrent 203, as shown in FIG. 2(c). If the photocurrent value is directly proportional to the intensity of the light reflected from document 5, a true photocurrent value will be obtained by subtracting photocurrent component 204 from photocurrent 203. However, the photocurrent value is not necessarily directly proportional to the intensity of the light reflected from the document. In order to obtain a correct photocurrent value, the bias light must be weakened and the response of the photoconductive elements cannot be speeded up sufficiently.